Method for cleansing metal-containing solutions

ABSTRACT

A method for cleansing metal-containing solutions, particularly, but not exclusively, process water which contains such metals as mercury, cadmium and/or lead, where the metals concerned are present in water-soluble form, such as the form of a chloride complex. The method is characterized by passing the solution through an ion-exchange material (4) which consists essentially of sulfhydrated cellulose, the metals being adsorbed in the sulfhydrated cellulose; regenerating the ion-exchange material (4) subsequent to the adsorption of a predetermined quantity of metal, by rinsing the ion-exchange material with an aqueous solution that contains hydrochloric acid (HCl) at a concentration of at least about 1 molar HCl; and precipitating the metals concerned in sulphide form, in a known manner, from the liquid that is eluted from the ion-exchange material (4) in the aforementioned rinsing process. 
     The invention also relates to apparatus for carrying out the method.

The present invention relates to a method for cleansing metal-containingsolutions that are rich in salt, and apparatus herefor.

It is known that certain metals, primarily mercury, cadmium and lead,have a serious negative effect on different ecological systems.Consequently, it is necessary to find ways and means of extracting suchdeleterious substances from, inter alia, different types of flue gasesand process water.

Swedish patent specification No. 440 608the corresponding U.S. Pat. No.4,583,999 dated Apr. 22, 1986, Swedish patent application 860 2495-7 andSwedish patent application No. 860 2989-9 teach gas-cleansing methods inwhich, inter alia, mercury is extracted. According to these methodsmercury, among other substances, is captured in an acid condensate, themercury forming a chloride complex. Cadmium is also a substance which isextracted from flue gas in this way.

Mercury has a great tendency towards forming complexes in water withboth organic and inorganic ligands. In order to extract mercury from acomplex aqueous solution, it is necessary to add an agent with which themercury present will form a still more stable compound. Inorganic andorganic sulphides, such as TMT 15 or Na₂ S, can both be used for thispurpose. The resultant mercury sulphide precipitate, however, has anextremely fine granular form and is consequently difficult to separate.

The extent to which the metal sulphides can be extracted in practice iscontingent on the efficiency of those parts of the cleansing process inwhich the metal precipitates are separated from the system. The resultsobtained hitherto from such systems indicate that the residual contentsare normally so high as to require access to large water recipients inorder not to create ecological effects.

The above applies both to water that has become contaminated when usedin gas cleansing processes, and other industrial process water.

Consequently, one problem is to achieve a sufficiently high degree ofseparation or extraction of the mercury-containing metal precipitates.

Another problem resides in the very large volumes of water used in knowncleansing systems. Consequently, it is desirable to be able toconcentrate the mercury compounds so that they are present in a smallervolume of water prior to their final extraction from the system.

The same applies to other metals, such as cadmium and lead.

These problems are solved by the present invention, which provides asimple method and simple apparatus for extracting mutually differentmetals, primarily mercury, cadmium and lead.

Accordingly, the present invention relates to a method for cleansingmetal-containing solutions, particularly process water that containsmetals such as mercury, cadmium and/or lead, where the metal concernedis in a water-solution form, such as the form of a chloride complex, themethod being characterized by passing the solution through anion-exchange material which consists essentially of sulfhydratedcellulose, said metals being adsorbed in the sulfhydrated cellulose; byregenerating said ion-exchange material subsequent to having adsorbed agiven quantity of metal, wherein an aqueous solution containinghydrochloric acid (HCL) having a concentration of at least about 1 molarHCL is flushed or rinsed through said ion-exchange material; and bycausing, in a known manner, the metal concerned to precipitate in asulphide form from the liquid eluted from the ion-exchange material whenrinsing said material.

The invention also relates to apparatus of the kind by which theaforementioned method may be practised, namely apparatus for cleansingmetal-containing solutions, particularly, but not exclusively, processwater which contains such metals as mercury, cadmium and/or lead, wherethe metals concerned are present in a water-soluble form, such as in achloride complex form, and wherein the apparatus includes anion-exchange column which is provided with an inlet pipe forcontaminated liquid and an outlet pipe for cleansed liquid, and whichcontains an ion-exchange material which is comprised substantially asulfhydrated cellulose.

The invention will now be described in more detail, partly withreference to an exemplifying embodiment of an apparatus illustrated inthe accompanying drawing, in which

FIG. 1 is a schematic cross-sectional view of an apparatus constructedin accordance with the invention;

FIG. 2 is a plan view of a cassette according to the invention;

FIG. 3 is a sectional view taken on the line A-A in FIG. 2; and

FIG. 4 is a side view of the central part of the apparatus illustratedin FIG. 1.

The present invention is particularly suited for the extraction of oneor more of the metals mercury, cadmium, zinc and lead with the pertinentmetal being in a water-soluble form. When the invention is used forcleansing flue gases, the metal concerned will normally have the form ofa chloride complex.

According to the invention, an aqueous solution containing said complexis passed through an ion-exchange material which consists substantiallyof sulfhydrated cellulose.

The ion-exchange material is manufactured in the following manner. Thestarting material is cellulose (C₆ H₁₂ O₆), which may be either in loosefiber form or spun or otherwise shaped into a coherent structure.

The cellulose is required to be dry and is treated with a mixture ofthioglycolic acid, acetic anhydride, acetic acid, concentrated sulphuricacid and de-ionized water. The mixture may contain, for example, 50mlthioglycolic acid (HSCH₂ CO₂ H) 35ml acetic anhydride ((CH₃ CO)₂ O),16ml acetic acid (CH₃ CO₂ H), 0.15ml sulphuric acid (H₂ SO₄), and 5mlwater for each 10g of cellulose present.

This solution is sieved through the cellulose for from 1 to 7 calendardays at a temperature of 40°-45° C.. The thus treated cellulose is thenwashed with de-ionized water and dried to dryness at 40°-45° C..

During this treatment, the acetic anhydride causes the OH-groups in thecellulose to be replaced with acetate groups. The thioglycolic acidcauses the SH-groups present to substitute said acetate groups.

Sulfhydrated cellulose has the formula C₆ H₆ (SH)₆. When a metal, suchas mercury for instance, comes into contact with the sulfhydratedcellulose, the mercury is bound to the SH-groups, with the mercury (Hg)replacing the hydrogen atom H.

In order to ensure the adsorption of mercury, the contaminated watercharged to the ion-exchanger is brought to a pH higher than about 1,preferably to a pH of about 2. Such metals as zinc, cadmium, lead orcopper are not adsorbed to any appreciable extent at this pH.

In order to adsorb these substances, the contaminated water charged tothe ion-exchanger must have a pH in the region of or slightly higherthan 3.

Generally speaking the ion-exchanger will adsorb different metals atdifferent pH values. Since the aforementioned metals have merely beenmentioned by way of example, it will be understood that the invention isnot restricted to the extraction of just these metals, and neither is itrestricted to contaminated water with just the aforesaid pH-values.

One of normal skill in this art is fully capable of adjusting the pH ofthe contaminated water to a level applicable to the metal or metals tobe adsorbed, with the aid of conventional analysis techniques. Accordingto the invention, the ion-exchange material is regenerated subsequent tohaving adsorbed a predetermined amount of metal. It can be mentioned byway of example that the ion-exchange material should be regenerated when50-70g of mercury has been adsorbed for each 1000g of ion-exchangematerial

The ion-exchange material is regenerated by rinsing the same with anaqueous solution that contains hydrochloric acid (HCl) at aconcentration of at least 2 molar HCl. Alternatively, the concentrationmay be 1 molar HCl, but then in combination with about 2 molar of commonsalt (NaCl). The acid concentration may also be higher, e.g. 3 molesHCl.

The amount of regenerating solution required is about 2-4 bed volumes.

The regenerating process is effected so that at such high acidstrengths, optionally in combination with a high proportion of salt,i.e. in the presence of other cations than Hg²⁺, the mercury isdisplaced from its position at the locations of the sulphur (S) of thesulfhydrated cellulose and returns to solution while being replaced withhydrogen (H) at the same time. The ion-exchanger can then be re-used.

The metal concerned is then precipitated in sulphide form from the watereluted from the ion-exchange material when rinsing the same. This isachieved, for instance, by adding TMT 15 or Na₂ S.

It may be convenient to filter the contaminated water through a bed offine sand for example, prior to charging contaminated water to theion-exchanger, in order to extract from the water substances suspendedtherein, and therewith prevent those substances from sludging-up theion-exchange material.

A very high degree of metal extraction is achieved in the ion-exchangerwhen practicing the present invention.

Furthermore, subsequent to regenerating the ion-exchange material theextracted metals are concentrated in a volume of liquid which isrelatively small when compared with the volume of liquid that has passedthrough the ion-exchanger.

The following facts can be mentioned by way of example.

Approximately 10kg of sulfhydrated cellulose are required to clean 100m³of liquid used to cleanse flue gases of their mercury content inaccordance with Swedish patent No. 440 608 (U.S. Pat. No. 4,583,999). Abed volume in this case may be 50 l. In order to regenerate such a bed,it is necessary to use 2-4 bed volumes of regenerating liquid, i.e. 100l for example.

Thus the mercury contained in 100m³ of liquid has been concentrated to aliquid volume of 100 l, i.e. the mercury has been concentrated 1000times.

As beforementioned, the invention also relates to apparatus forcleansing solutions which contain metals of the aforesaid kind.

One such apparatus is illustrated schematically in FIGS. 1-4. Theapparatus includes an ion-exchange column 1, which is provided with aninlet pipe 2 for contaminated liquid and an outlet pipe 3 for cleansedliquid. The ion-exchange column contains an ion-exchange material 4,which is substantially comprised of sulfhydrated cellulose.

The ion-exchange material 4 is carried by one or more cassettes 5 whichcomprise two mutually parallel net structures 6, 7 which sandwich theion-exchange material therebetween, thereby holding said material firmlyin place, and which are in turn carried by a frame 8. The ion-exchangematerial is preferably spun or pressed to a mechanical strengthsufficient for it to be retained by the net structures.

According to one preferred embodiment of the invention, the ion-exchangecolumn includes a plurality of cassettes 5. The apparatus also includesdevices for forcibly driving the contaminated liquid through theion-exchange material in the cassettes 5. These devices include a pump9, and also channels 10 which are so arranged that the contaminatedliquid is forced to pass the ion-exchange material in order to be ableto flow from the inlet 2 to the outlet 3.

For example, the ion-exchanger may have the construction illustrated inFIG. 1, in which a plurality of cassettes 5 are placed in mutualparallel relationship in two tiers mutually separated by a bearing frame11. The cassettes 5 are placed in a surrounding casing 12 or housing,the linear extension of which perpendicular to the paper in FIG. 1corresponds to the width of a cassette 5. The inlet pipe 2 is isolatedcentrally in the roof 13 of the housing, whereas two outlets 15, 16 areprovided at the outer extremities of the housing floor 14. These outletsare joined in the outlet pipe 3. The carrier frame 11 incorporates theaforesaid channels 10. The diameter of these channels are adapted so asto provide a throttling effect such that subsequent to entering theion-exchange column through the central inlet 2, the contaminated wateris divided into flows which pass respectively horizontally through thecassettes in the upper cassette tier and through the channels 10 to thecassettes located in the lower cassette tier, as indicated by the arrowsin FIG. 1. The liquid flowing through the channels 10 is forced outwardsin the lower tier of cassettes, as indicated by the arrows, and thendeparts through the outlets 15, 16.

In the illustrated embodiment of the apparatus each cassette 5 isslideably mounted in a pair of upper and lower rails 17, 18 of U-shapedcross-section, the frames 8 surrounding the cassettes being guided inthe rails at right angles to the plane of the paper in FIG. 1. Thisenables the cassettes to be changed quickly and easily.

FIG. 4 is a side view of the ion-exchange column and shows in brokenlines one cassette 5 of the upper tier and one cassette 5 of the lowertier. One side 19 of the column is preferably removable so as to enablethe cassettes to be inserted into and removed from the column in therespective directions of the arrows 20 and 21.

The ion-exchange column and the cassetees, inclusive of the netstructures, are made of an acid-proof material, preferably a suitableplastics material.

The contaminated liquid is charged to the apparatus through the inlet 2,as indicated by the arrow 22, and then passes through the ion-exchangematerial.

The cleansed liquid exiting through the outlet 3 is conducted to one ormore vessels (not shown), optionally to be cleansed of substances otherthan the metals extracted in the illustrated apparatus, prior toreleasing the cleansed liquid to a water recipient, as indicated by thearrow 23.

A two-way valve 24 is arranged downstream of the outlet 3. Connected tothe valve 24 is a discharge pipe 25 for cleansed liquid and a furtherdischarge pipe 26 which discharges into a vessel 27.

A further two-way valve 28 is arranged upstream of the pump 9 and hasconnected thereto the inlet pipe 2 and a further pipe 29, through whichregenerating liquid is passed as shown by the arrow 30.

When regenerating liquid is charged to the apparatus, the supply ofcontaminated liquid thereto is cut-off by the valve 28 and theregenerating liquid is pumped through the ion-exchange column anddischarged, via the valve 24 and the pipe 26, to the vessel 27, where itis collected for precipitation of metals in the manner described above.The liquid is then taken from the vessel 27 and passed through a pipe 31to a treatment vessel (not shown) for optional further treatment.

It is quite evident from the aforegoing that the present inventionsolves the two problems recited in the introduction.

It will be understood that the present invention is not restricted tothe aforedescribed embodiment or mixtures and that these may bemodified. Furthermore, the sulfhydrated cellulose may have a physicalform other than that described.

The invention shall not therefore be considered limited to theaforedescribed embodiment, and it shall be understood that modificationscan be made within the scope of the following claims.

We claim:
 1. A method for cleansing metal-containing solutions, whichcomprise a fluid solution selected from the group consisting of watercontaining contaminates from contaminated flue gases and contaminatedprocess water, said fluid solution containing at least one of the metalsfrom the group consisting of mercury, cadmium, zinc and lead, the metalsbeing present in water-soluble form, the method comprising: passing saidsolution through an ion-exchange material (4) which consists essentiallyof sulfhydrated cellulose, said metals being absorbed in thesulfhydrated cellulose; regenerating the ion-exchange material (4)subsequent to the absorption of a predetermined quantity of metal, byrinsing the ion-exchange material with an aqueous solution that containshydrochloric acid at a concentration of at least about 1 molar HC1 andprecipitating the metals in sulphide form from the liquid that is elutedfrom the ion-exchange material in said rinsing process.
 2. A methodaccording to claim 1, wherein the solution which is treated containsmercury contaminated water, further comprising adjusting the pH of thecontaminated fluid solution charged to the ion-exchange material to avalue within the range of from about 1 to about 2, in order to extractmercury from said water.
 3. A method according to claim 1, wherein thesolution which is treated is a contaminated liquid solution, furthercomprising adjusting the pH of the contaminated liquid solution chargedto the ion-exchange material to a value of at least
 3. 4. A methodaccording to claim 1, wherein the aqueous solution used to regeneratethe ion-exchange material has a concentration of at least 2 molar HC1.5. A method as defined in claim 1, 2, 3 or 4, wherein said water solubleform of said at least one metal selected from the group consisting ofmercury, cadmium, zinc or lead is a chloride complex.
 6. A methodaccording to claim 1, wherein the aqueous solution used to regeneratethe ion-exchange material has a concentration of at least 1 molar HC1and a concentration of about 2 molar of NaCl.